Jason Stryker was a student of Professor X, who has the ability to create illusions.
William Stryker chose to not to kill his son, Jason (who himself is partially modeled after Mastermind, another X-Men foe), at birth, His father sent him to Xavier's school in hopes of curing him, regarding mutation as a disease that must be cured. Xavier had no interest or belief in 'curing' mutants, which angered his father. Additionally, Jason continued to grow angry, resentful, and vindictive towards his parents; he tortured them by planting illusions in their brains until his mother committed suicide by drilling into her own brain in order to "bore the images out." his father then gave his son a lobotomy to make him more docile.
Jason created the illusion of a little girl in the Mansion X on his father orders to get Xavier to find all the mutants by using Cerebro II to do so. Magneto changed this with the help of Mystique to shapeshift into Stryker to make Jason change Xavier to find all humans instead.
Nightcrawler teleported Storm and himself into where Xavier and Jason were. Jason created an illusion of a little girl to stop them, but Storm used her ability to stop Jason, who turned his illusion off and Nightcrawler teleported, Storm, Xavier and himself out of there, leaving Jason to die.